The present invention relates to a protective circuit for preventing an amplifier circuit from being destroyed.
When various amplifier circuits, particularly power amplifier circuits in audio apparatus are subject to an excessively boosted voltage of a power source, the components of the circuits may possibly be damaged or destroyed. Some proposals have been made to protect the circuit from such damage.
Referring now to FIG. 1, there is shown an amplifier circuit into which a conventional protective circuit is incorporated. Reference numeral 2 designates a pre-amplifier. Numeral 4 denotes a power amplifier. Numeral 6 designates a protective circuit for protecting the power amplifier stage from being damaged or destroyed. Numeral 8 is a load such as a speaker and numerals 10, 12, 14 and 16 are DC blocking capacitors for blocking DC components from passing. Assume that a power voltage V.sub.CC is excessively boosted and zener diode 18 becomes conductive. In this case, current passes through resistors 20 and 22 so that a potential difference appears across resistors 20 and 22. When the potential difference across the resistor 22 reaches a threshold voltage between the base-emitter of a transistor 24, the transistor 24 is turned on. The turning on of transistor 24 blocks the input signal delivered through the capacitor 12 from going to the power amplifier 4. That is, the input signal flows to ground through the collector-emitter path of the turned-on transistor 24. In this manner, the transfer of the input signal to the power amplifier stage 4 is blocked when power source voltage is raised too high. As a result, destruction of the power amplifier 4 is prevented when there is excessive boosting of power source voltage.
As described and shown, the protective circuit 6 is connected to the power amplifier circuit 4, through the blocking capacitor 14. Therefore, it is impossible to instantaneously eliminate the input signal. In consequence, it frequently fails to protect the power amplifier circuit 4 when the power source voltage is rapidly raised.
Further, it is impossible to eliminate the collector current flowing into the output transistor of the power amplifier 4, when no input signal is present. Even if the power source voltage is gently raised, there is a possibility that the transistor 24 will consume an excessive power expressed by the product of the collector current flowing at no input signal and the power source voltage.